It is known that phenolic resins are virtually impossible to surpass by other synthetic resins in chemical resistance, in water and solvent resistance, and in corrosion resistance (Ullmann's Encyclopedia of Industrial Chemistry, Paints and Coatings, vol. A18, (1991), p. 418). Particularly in the field of interior protective coating materials for coating metal packaging forms made from tinplate, aluminum, and sheet steel, heat-curable phenolic resins have proved to be outstanding.
The prime function of interior coatings on containers intended for use for keeping beverages and foodstuffs is to protect the packaging against corrosion by the contents and, conversely, to preserve the contents against the effect of metal ions and the associated taste impairment.
Very largely pore-free film formation, sufficient hardness, flexibility, and scratch resistance, and also firm adhesion and sterilization resistance are further important properties vital to interior protective coatings.
In addition, the composition of the coating material is subject to the statutory regulatory provisions of the Bundesgesundheitsamt in Germany and to approval in the USA by the FDA (21 CFR § 175.300—regulation).
In order to comply with the profile of requirements of an interior coating, however, phenolic resins cannot be used as sole binders. To date the phenolic resins have been used only in the presence of elasticizing resins, particularly epoxy resins.
From the literature it is known that phenolic resins are plasticized or flexibilized using relatively high molecular mass epoxy resins of type 7 or 9. These types of epoxy resin exhibit not only good adhesion to steel, tinplate, aluminum, and other common container materials but also good resistance to dilute acids (e.g., acetic acid, lactic acid, and carbonic acid) and sulfur. They are additionally stable to pasteurization and to sterilization. The type 7 or 9 epoxy resins used, however, include mass fractions of up to 5% each of BADGE (bisphenol A diglycidyl ether) and free bisphenol A. These two compounds, however, are not unobjectionable from the standpoint of health. Furthermore, for sufficient curing, leading to the sufficient resistance to solvents, for example, that is required for the coating film, these coating systems require high baking temperatures and long cure times. Accordingly, more reactive binder systems are desirable for the user.
The object is therefore to provide binders for metal container coating which can be used inter alia as sole binders and which do not have the above disadvantages, or at least only to a reduced extent.